Al-Ali Hassan, Ding Ying, Slepak Tatiana, Wu Wei, Sun Yan, Martinez Yania, Xu Xiao-Ming, Lemmon Vance P, Bixby John L
Miami Project to Cure Paralysis, University of Miami Miller School of Medicine, Miami, Florida 33136.
Peggy and Harold Katz Family Drug Discovery Center, University of Miami Miller School of Medicine, Miami, Florida 33136.
J Neurosci. 2017 Jul 26;37(30):7079-7095. doi: 10.1523/JNEUROSCI.0931-17.2017. Epub 2017 Jun 16.
The mammalian target of rapamycin (mTOR) positively regulates axon growth in the mammalian central nervous system (CNS). Although axon regeneration and functional recovery from CNS injuries are typically limited, knockdown or deletion of PTEN, a negative regulator of mTOR, increases mTOR activity and induces robust axon growth and regeneration. It has been suggested that inhibition of S6 kinase 1 (S6K1, gene symbol: RPS6KB1), a prominent mTOR target, would blunt mTOR's positive effect on axon growth. In contrast to this expectation, we demonstrate that inhibition of S6K1 in CNS neurons promotes neurite outgrowth by twofold to threefold. Biochemical analysis revealed that an mTOR-dependent induction of PI3K signaling is involved in mediating this effect of S6K1 inhibition. Importantly, treating female mice with PF-4708671, a selective S6K1 inhibitor, stimulated corticospinal tract regeneration across a dorsal spinal hemisection between the cervical 5 and 6 cord segments (C5/C6), increasing axon counts for at least 3 mm beyond the injury site at 8 weeks after injury. Concomitantly, treatment with PF-4708671 produced significant locomotor recovery. Pharmacological targeting of S6K1 may therefore constitute an attractive strategy for promoting axon regeneration following CNS injury, especially given that S6K1 inhibitors are being assessed in clinical trials for nononcological indications. Despite mTOR's well-established function in promoting axon regeneration, the role of its downstream target, S6 kinase 1 (S6K1), has been unclear. We used cellular assays with primary neurons to demonstrate that S6K1 is a negative regulator of neurite outgrowth, and a spinal cord injury model to show that it is a viable pharmacological target for inducing axon regeneration. We provide mechanistic evidence that S6K1's negative feedback to PI3K signaling is involved in axon growth inhibition, and show that phosphorylation of S6K1 is a more appropriate regeneration indicator than is S6 phosphorylation.
雷帕霉素的哺乳动物靶点（mTOR）正向调节哺乳动物中枢神经系统（CNS）中的轴突生长。尽管中枢神经系统损伤后的轴突再生和功能恢复通常有限，但mTOR的负调节因子PTEN的敲低或缺失会增加mTOR活性，并诱导强大的轴突生长和再生。有人提出，抑制mTOR的一个重要靶点S6激酶1（S6K1，基因符号：RPS6KB1）会削弱mTOR对轴突生长的积极作用。与这一预期相反，我们证明，抑制中枢神经系统神经元中的S6K1可使神经突生长增加两倍至三倍。生化分析表明，PI3K信号的mTOR依赖性诱导参与介导S6K1抑制的这一作用。重要的是，用选择性S6K1抑制剂PF-4708671治疗雌性小鼠，可刺激皮质脊髓束在颈5和颈6脊髓节段（C5/C6）之间的背侧半脊髓横断处再生，在损伤后8周时，损伤部位以外至少3毫米处的轴突数量增加。同时，PF-4708671治疗产生了显著的运动功能恢复。因此，对S6K1进行药物靶向可能是促进中枢神经系统损伤后轴突再生的一种有吸引力的策略，特别是考虑到S6K1抑制剂正在针对非肿瘤适应症进行临床试验评估。尽管mTOR在促进轴突再生方面的功能已得到充分确立，但其下游靶点S6激酶1（S6K1）的作用尚不清楚。我们使用原代神经元进行细胞试验，证明S6K1是神经突生长的负调节因子，并使用脊髓损伤模型表明它是诱导轴突再生的一个可行的药物靶点。我们提供了机制证据，表明S6K1对PI3K信号的负反馈参与轴突生长抑制，并表明S6K1的磷酸化比S6的磷酸化是更合适的再生指标。
